1. Field of the Invention
The present invention relates to a squirrel-cage induction motor and, more particularly, to improvements in the method of controlling the starting current and speed adjustment for a squirrel-cage induction motor.
2. Description of the Prior Art
In the squirrel-cage induction motor, in general, three-phase ac current is supplied to the stator windings to generate a rotating magnetic field, and the interaction of the rotating magnetic field and the current flowing through the squirrel-cage conductors of the rotor generates a torque, which drives the rotor core for rotation.
A star-delta switching starter, a reactor starter and a starting compensator are conventional starting current control means for controlling the starting current of the squirrel-cage induction motor. However, these conventional starting current control means are merely capable of stepped control of the starting current and, once set, the desired starting current cannot be changed.
A secondary resistance control method employing a wound-rotor induction motor is a starting current control means capable of continuously controlling the starting current. However, requiring a wound rotor, the secondary resistance control method makes the construction of the motor complicated, hence, the motor is expensive, needs additional components such as a metallic resistor or a liquid resistor, and a controller for regulating the resistor. Furthermore, the secondary resistance control method suffers from a significant power loss in the resistor.
Improving the characteristics of the induction motor so that the induction motor can be started by a low starting current without using any special starter is another starting current control means. However, reduction in the starting current affects the torque characteristics of the induction motor adversely and deteriorates the operating characteristics of the induction motor. Accordingly, the starting current can be suppressed, at the lowest, to a starting current on the order of 350 to 400% of the rated current. In this case, a transformer of a power capacity several times the working power must be installed to suppress voltage drop of the transformer.
The applicant of the present invention previously proposed a squirrel-cage induction motor comprising a stator divided into two portions disposed side by side, and squirrel-cage rotor disposed opposite to the stator, in which the currents flowing through the squirrel-cage windings are suppressed to control the starting current continuously over a wide range by shifting the relative position of the two portions of the stator in circumferential directions to shift the phase of the electromotive forces induced in the squirrel-cage windings.
FIG. 1 shows a squirrel-cage induction motor previously proposed by the applicant of the present invention in Japanese Patent Laid-open (Kokai) No. 59-191461. Shown in FIG. 1 are brackets 1, bearings 2, a rotary shaft 3 supported in the bearings 2 on the brackets 1, a rotor core 4 fixedly mounted on the rotary shaft 3, rotor conductors 5 provided on the rotor core 4, end rings 6 provided respectively at the opposite ends of the rotor conductors 5 and shorting the ends of the rotor conductors 5 to form squirrel-cage windings, a frame 9, a first stator A fixed to the frame 9 and having stator windings 8A, a second stator B having stator windings 8B and mounted on the frame 9 so as to be turned in circumferential directions, slide rings 10 attached to the circumference of the second stator B, guide rings 11 attached to the inner surface of the frame 9 and respectively slidably receiving the slide rings 10 therein, a turning rod 12 fixed to the second stator B so as to project from the circumference of the frame 9, a hydraulic actuator 13 comprising a piston connected to the turning rod 12, a cylinder unit 15 receiving the piston 14 therein for axial movement. Working fluid is supplied through either of inlet ports 16 and 17 into the cylinder unit 15 to turn the second stator B in either circumferential direction through the piston 14 and the turning rod 12.
The second stator B can be turned within an angular range corresponding to one pole pitch of magnetic poles produced by supplying a three-phase alternating current to the windings.
Three phase alternating currents are supplied to the first stator A and the second stator B so that rotating magnetic fields of the same rotating direction are produced by the first stator A and the second stator B. Then, as shown in FIG. 2, a current I.sub.A induced by the rotating magnetic field of the first stator A and a current I.sub.B induced by the rotating magnetic field of the second stator B flow through the squirrel-cage windings. When the two stators A and B are positioned so that the poles produced by the three-phase alternating currents are in the same phase, the currents I.sub.A and I.sub.B are the same in magnitude and the direction of flow as shown in FIG. 2 and a resultant current I.sub.C flows through the squirrel-cage windings of the rotor.
When the second stator B is shifted relative to the first stator A in the circumferential direction by an angle corresponding to half the pole pitch, currents I.sub.A and I.sub.B as shown in FIG. 3 flow through the squirrel-cage windings and thereby a reduced and deformed resultant current I.sub.C as shown in FIG. 3 flows through the squirrel-cage windings of the rotor.
When the second stator B is shifted relative to the first stator A in the circumferential direction by an angle corresponding to one pole pitch, currents I.sub.A and I.sub.B as shown in FIG. 4 flow through the squirrel-cage windings, in which the currents I.sub.A and I.sub.B cancel each other and hence the resultant current I.sub.C is approximately zero.
Accordingly, the squirrel-cage induction motor has torque-speed characteristics as shown in FIG. 5(a) and current-speed characteristics as shown in FIG. 5(b). Torques T.sub.1 to T.sub.4 in FIG. 5(a) correspond respectively to starting currents I.sub.1 to I.sub.4 in FIG. 5(b).
This known squirrel-cage induction motor thus constructed has problems that is difficult to control the relation between the displacement of the second stator and the characteristics required by load, the response characteristics is unsatisfactory, the squirrel-cage induction motor cannot be incorporated into an automatic control system because the second stator cannot manually be shifted for phase control when the hydraulic mechanism malfunctions.